. >> madam chairwoman, delegates, i accept your nomination for president of the united states. >> reporter: still you couldn't help but think, what a difference four years have made. so much grayer now. the toll the presidency takes. the youthful aura that clung to barack obama in 2008 has been burned away by the searing pressures of power. he spoke tonight a man of hard and bitter experience. a survivor. >> i recognize that times have changed since i first spoke to this convention. >> reporter: that's why vice president joe biden went all out, too. >> we can now proudly say what we have heard me say the last six months. osama bin laden is dead, and general motors is alive! >> reporter: the president did something else. he addressed directly the frustrated hopes and mixed feelings about his leadership out across the country. >> now i won't pretend the path i'm offering is quick or easy. >> reporter: this was really the president's biggest challenge, the biggest challenge in his campaign. to overcome the disappointments and doubts among so many americans who voted for him in 2008. and give